This invention relates to direct sequence spread spectrum receivers and, more particularly, to interaction between delay searcher and delay trackers for new delay assignment to a rake receiver.
In a typical RF communication system, a transmitted signal may travel from a transmitter to a receiver over multiple paths, for example a direct path and also a reflected path. Each path may be considered a separate channel which is subject to the effects of fading, dispersion, etc. Moreover, the combination of signals at the receiver can result in additional fading. Such operating environments are known as multipath fading environments. Direct sequence spread spectrum (DS-SS) receivers can operate in multipath fading environments. A DS-SS receiver typically includes a rake receiver, which demodulates a received signal using plural demodulation xe2x80x9cfingersxe2x80x9d, often referred to as rake fingers. Each rake finger demodulates the component signal from a number of the channel paths (such component signal referred to as a multipath component). The outputs of the rake fingers are combined for improved performance.
With multipath channels a transmitted signal arrives in components, with each component having a different delay. The components can be distinguished and resolved if the delays are of sufficient duration. However, in order to demodulate the signals, the rake receiver must know the delay of each channel path.
Typically, a rake receiver operates in conjunction with a delay searcher and a delay tracker. The delay searcher analyzes a received signal and finds the delays. These delays are assigned to the rake fingers. However, in mobile telecommunications the channels may be subject to additional fading due to the motion of the receiver. A delay tracker tracks the delays assigned by the searcher between channel searches. Thus, while the searcher looks over a wide range of delays, the trackers look over a smaller range surrounding the assigned delays.
With such a configuration, problems may result during reassignment after a new search. One problem relates to loss of accuracy after the new search. Typically, a delay searcher uses less resolution than a delay tracker. When a new search is performed, the delay searcher may find a path with a delay that is close, but not exactly the same as that being tracked by the tracker. The path may actually be the same being tracked by the delay tracker. The difference in sensed delay is due to the lesser resolution of the delay searcher. Nevertheless, the delay searcher would assign the new delay to the rake finger, resulting in loss of accuracy until such time as the tracker subsequently adjusts the new delay.
Another problem relates to unnecessary relocation of rake finger delays. Typically, a delay searcher will assign a delay associated with the earliest arriving channel path to the first rake finger. Likewise, the next arriving channel path is assigned to the next rake finger, and so on. Due to fluctuation in relative signal strength of the changing channel paths, this can cause the same channel path to be reassigned to a different rake finger after subsequent searches. This reassignment takes time, and data may be lost during reassignment of the same channel path to a different rake finger.
The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.
In accordance with the invention there is disclosed an apparatus and method using delay searcher and delay tracker interaction for new delay assignment to a rake receiver.
Broadly, there is disclosed herein a direct sequence spread spectrum receiver for operating in a multipath fading environment comprising a rake receiver having plural rake fingers. Each rake finger demodulates a received signal component from one of plural channel paths (i.e., a multipath component). The outputs of the plural rake fingers are combined. Each rake finger utilizes a select assigned delay to synchronize to a delay of the channel path to which it is assigned. A searcher periodically performs a channel search on the received signal to detect new delays of strongest paths in the channel. Plural trackers, one for each channel path, adjust the select assigned delays between searches performed by the searcher. A delay controller is operatively coupled to the searcher and the tracker. The delay controller compares new delays of the strongest paths from the searcher to the select assigned delays and reassigns one of the select assigned delays with one of the new delays only if the new delay differs from the one select assigned delay more than a preselect minimum amount.
It is a feature of the invention that the delay controller compares the detected delay of each of the strongest paths to each of the select assigned delays to determine nearest new delay to one of the select assigned delays. The delay controller determines if the determined nearest new delay is for the same path as the select assigned delay if the detected delay for the nearest new delay differs from the one select assigned delay less than the preselect minimum amount.
It is another feature of the invention that the delay controller reassigns the select assigned delays with a closest one of the detected new delays.
In accordance with another aspect of the invention, there is disclosed a direct sequence spread spectrum receiver for operating in a multipath fading channel including a rake receiver having plural rake fingers. Each rake finger demodulates a received signal from one of plural channel paths. The outputs of the plural rake fingers are combined. Each rake finger uses a select assigned delay to synchronize to a delay of the channel path to which it is assigned. A searcher periodically performs a channel search on the received signal to detect new delays of strongest paths in the channel. A delay controller is operatively coupled to the searcher. The delay controller compares new delays of the strongest paths from the searcher to the select assigned delays, and reassigns the select assigned delays with a closest one of the new delays.
In accordance with another aspect of the invention, there is disclosed a method of processing a received signal in a multipath fading channel comprising the steps of demodulating the received signal from each of plural channel paths utilizing a select assigned delay synchronized to a delay of one of the channel paths, periodically performing a channel search to detect new delays of strongest paths in the channel, adjusting the select assigned delays between channel searches, and comparing new delays of the strongest paths to the select assigned delays and reassigning one of the select assigned delays with one of the new delays only if the new delay differs from the one select assigned delay more than a preselect minimum amount.
There is disclosed in accordance with yet a further aspect of the invention a method of processing a received signal in a multipath fading channel, comprising the steps of demodulating the received signal from each of plural channel paths utilizing a select assigned delay to synchronize to a delay of one of the channel paths, periodically performing a channel search to detect new delays of strongest paths in the channel, and comparing new delays of the strongest paths to the select assigned delays and reassigning the select assigned delays with a closest one of the new delays.
Further features and advantages of the invention will be readily apparent from the specification and from the drawings.